Leica Sport Optics

Great Grey Owl / Strix nebulosa
A primer for binoculars We hope that the following glossary of main terms and
quality criteria for the evaluation of binoculars and spotting scopes will be helpful to you
in making a purchasing decision. In addition, here are authorized Leica dealers and representatives
around the world who will be pleased to assist you. They have a large selection
of products and they will gladly present you your favorite model.
Technical binocular terms
Magnification Every binocular is specified by two numbers, of which the first one always indicates the magnification.
Magnification tells us how much closer an object appears to the observer when viewed through the binocular. For example :
At 8 x magnification, a bird that is 328 feet/100 m away appears to be only 41 feet/12.5 m (328 ft : 8 = 41 ft) away.
Front lens diameter The second characteristic number of a binocular is the front lens diameter
(the entry pupil of the binocular) stated in millimeters. A binocular with the designation 10 x 50
has a front lens diameter of 50 mm. The larger the front lens diameter, the more light can be gathered
by the binocular. If observation is to be conducted at twilight or at night, the ideal choice is
a front lens diameter of 42 or 50 mm. Binoculars with a front lens diameter of 20 to 32 mm are
suited primarily for observation in daylight.
Twilight factor The twilight factor is a computed number
that describes the theoretical performance capability of
a binocular at twilight. It is computed by taking the square
root of the product of the magnification factor and the
front lens diameter. The twilight factor is a purely mathematical
value that says nothing about the optical quality
of a binocular, such as contrast, neutral color rendition,
resolving power, etc.
Objective field of view This value defines the width of
the field of view at a distance of 1000 meters. Usually,
the greater the magnification, the smaller the field of view.
Binoculars with a large field of view enable the observer
to cover a large area and to follow moving subjects, such
as a flock of birds, conveniently. A significant quality
criterion of Leica binoculars is an image that is a field of
view that is crisp and sharp to its very edges and that is
rich in contrast.
7x
10 x
30 x
86 / 87
Exit pupil The two bright circular areas that can be seen when one looks at the
eyepiece of a binocular from a distance of approximately 12 inches/30 cm. (Exit
pupil = front lens diameter in mm : magnification factor). An indication of quality is
that these areas should be exactly circular and that they should have sharp edges.
The pupil of the eye changes its diameter in accordance with the light intensity (small
in bright light, large in the dark). For example, a 40-year old observer typically has a
maximal pupil diameter of 6 mm. A rule of thumb for comfortable observation is that
the exit pupil of a binocular should be at least as large as the pupil of the eye at its
largest diameter. The exit- and entry pupils are not, however, the only decisive quality
characteristics for the evaluation of image brightness : Factors such a contrast,
resolving power and light transmission of a binocular or spotting scope are just as
important.
Example LEICA TRINOVID 10 x 42 BN
Magnification = 10 x, front lens diameter = 42 mm
front lens diameter
Exit pupil = = 4.2 mm
magnification
Twilight factor = front lens diameter x magnification factor = 20.5
60 x